They won't get split up - it was stated during the last round of this OU and UT wanted to be in the same division so they could keep the Red River Showdown and not have to play each other in a conference championship game. They only want to play each other once per year.
The ACC has "protected rivalry" games where two teams that are not in the same division still play each other every year. I expect that the Big 12 would develop some sort of scheduling system that would allow for that.